A

Step 1: Understand that the barometer is filled with mercury and is closed at point A by a cork. At this point, the mercury is at a certain height due to atmospheric pressure acting on it.
Step 2: When the cork at A is removed, the pressure above the mercury column will equalize with the atmospheric pressure.
Step 3: The atmospheric pressure will push down on the mercury, leading to a change in height. The mercury in the barometer will fall.
Thus, when the cork is removed from opening A, the mercury level will drop, demonstrating that the atmospheric pressure is balancing the weight of the mercury column.